Product evolution has resulted in a physical division of a cellular base station into two spatially separated primary components, a remote radio head and a baseband unit. The remote radio head implements the air interface and is relieved of backhaul responsibilities by the baseband unit. The remote radio head is located according to radio coverage concerns, whereas the baseband unit may be located elsewhere because it is fragile or for easy maintenance.
A baseband unit may serve multiple collocated remote radio heads, thereby reducing rollout costs and power consumption. If the baseband unit is multi-mode or multi-band, it may serve multiple cellular networks that cover the same vicinity by serving a different remote radio head for each cellular network. Collocation of radio transmitters of different cellular networks may cause inter-band interference, especially if the radio bands of the two cellular networks are nearly spectrally adjacent. For example a commercial cellular network and an emergency response cellular network may have collocated transmitters with adjacent bands that cause passive intermodulation interference.
Saving cost by accommodating multiple bands with the same equipment also applies to the design of a remote radio head. A multi-band remote radio head transmits for multiple cellular networks if fed either by a single multi-band baseband unit for all of the cellular networks or by multiple single-band baseband units with each unit serving a separate cellular network. Despite the cost savings, a multi-band remote radio head does not alleviate inter-band interference of collocated transmitters.
Inter-band interference by two collocated transmitters occurs when both transmitters transmit simultaneously. Hence coordination between collocated transmitters can avoid inter-band interference by scheduling alternating transmission periods such that the transmitters take turns transmitting. Due to its centralized control of its antennas, a multi-band remote radio head is well suited to achieve alternating transmission periods.